Superconductivity mostly appears in high carrier density systems and sometimes exhibits common phase diagrams in which the critical temperature $T_{\mathrm{c}}$ continuously develops with carrier density. Superconductivity enhanced in the lightly doped regime has seldom been reported, although it is an ideal direction towards the crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) limits, where the behavior of Cooper pairs changes dramatically. Here we report transport properties and superconducting gaps in single-crystalline lithium-intercalated layered nitrides $({\mathrm{Li}}_x\mathrm{HfNCl}$ and ${\mathrm{Li}}_x\mathrm{ZrNCl})$ down to the low-doping regime, enabled by a combination of ionic gating device and tunneling spectroscopy. Upon the reduction of doping, both systems display the increase of $T_{\mathrm{c}}$, up to 24.9 K especially in ${\mathrm{Li}}_x\mathrm{HfNCl}$, with the concomitant enhancement of two-dimensionality leading to a pseudogap state below 35.5 K as well as the increase of superconducting coupling strengths $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}$ reaching 5.9. Such behavior in the carrier density region as low as ${10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$ indicates that lightly doped two-dimensional superconductors exhibit the unprecedented nature that is distinct from that in conventional superconductors, and offer a new route to access BCS-BEC crossover.

The establishment of a rigorous theory on the thermodynamics of light management in photovoltaics that accommodates various loss mechanisms as well as wave-optical effects in the absorption and reemission of light is at stake in this contribution. To this end, we propose a theoretical framework to calculate the open-circuit voltage enhancement resulting from photon recycling ($\mathrm{\Delta }{V}_{\mathrm{oc}}^{\mathrm{PR}}$) with rigorous wave-optical treatment. It can be applied to both planar thin-film and nanostructured single-junction solar cells. We derive an explicit expression for $\mathrm{\Delta }{V}_{\mathrm{oc}}^{\mathrm{PR}}$, which reveals its dependence on internal quantum luminescence efficiency, parasitic reabsorption, and photon escape probabilities of reemitted photons. While the internal quantum luminescence efficiency is an intrinsic material property, both latter quantities can be determined rigorously for an arbitrary solar cell architecture by three-dimensional electrodynamic dipole emission calculations. We demonstrate the strengths and validity of our framework by determining the impact of photon recycling on the $V_{\mathrm{oc}}$ of a conventional planar organometal halide perovskite thin-film solar cell and compare it to established reference cases with perfect antireflection and Lambertian light scattering. Our calculations reveal $\mathrm{\Delta }{V}_{\mathrm{oc}}^{\mathrm{PR}}$ values of up to 80 mV for the considered device stack in the absence of angular restriction and up to 240 mV when the escape cone above the cell is restricted to ${\theta }_{\mathrm{out}}=2.5^{\circ }$ around the cell normal. These improvements impose severe constraints on the parasitic absorption as a parasitic reabsorption probability of only 2% reduces the $\mathrm{\Delta }{V}_{\mathrm{oc}}^{\mathrm{PR}}$ to 100 mV for the same angular restriction. Our work here can be used to provide design guidelines.

We explore theoretically the influence of Fano interference in the so-called Majorana oscillations in a T-shaped hybrid setup formed by a quantum dot (QD) placed between conducting leads and side coupled to a topological superconducting nanowire (TSNW) hosting zero-energy Majorana bound states (MBSs) at the ends. Differential conductance as a function of the external magnetic field reveals oscillatory behavior. Both the shape and amplitude of the oscillations depend on the bias voltage, degree of MBSs nonlocality, and Fano parameter of the system determining the regime of interference. When the latter is such that direct lead-lead path dominates over lead-QD-lead path and the bias is tuned in resonance with QD zero energy, pronounced fractional Fano-like resonances are observed around zero bias for highly nonlocal geometries. Further, the conductance profiles as a function of both bias-voltage and QD energy level display “bowtie” and “diamond” shapes, in qualitative agreement with both previous theoretical and experimental works. These findings ensure that our proposal can be used to estimate the degree of MBS nonlocality, thus allowing us to investigate their topological properties.

The parquet equations are a self-consistent set of equations for the effective two-particle vertex of an interacting many-fermion system. The application of these equations to bulk models is, however, demanding due to the complex emergent momentum and frequency structure of the vertex. Here, we show how a channel decomposition by means of truncated unities, which was developed in the context of the functional renormalization group to efficiently treat the momentum dependence, can be transferred to the parquet equations. This leads to a significantly reduced complexity and memory consumption scaling only linearly with the number of discrete momenta. We apply this technique to the half-filled repulsive Hubbard model on the square lattice and present approximate solutions for the channel-projected vertices and the full reducible vertex.

Based on Bruggeman's symmetric effective-medium formula and an explicit expression derived for the temperature evolution of the volume fractions of the metallic and isolating domains appearing during the heating and cooling of ${\mathrm{VO}}_2$, respectively, we develop a model to describe the hysteresis of its electrical and thermal conductivities as well as of its specific heat capacity. The model takes into account the average value and standard deviation of the transition temperatures of the individual domains, as well as their activation energies, which represent the driving force for the existence of the ${\mathrm{VO}}_2$ hysteresis. It is shown that the model's predictions driven by these three parameters related to the microstructure of ${\mathrm{VO}}_2$ are in good agreement with robust experimental data. Furthermore, as these parameters are intrinsically correlated to the doping, defect, strain, and interface effects of ${\mathrm{VO}}_2$, the proposed model enables the seamless integration of these effects, and therefore, its predictions are also expected to be useful for describing the thermal and electrical properties of composites based on ${\mathrm{VO}}_2$.

Ballistic injection in a nanodevice is a complex process where electrons can be either transmitted or reflected, thereby introducing deviations from the otherwise quantized conductance. In this context, quantum rings (QRs) appear as model geometries: in a semiclassical view, most electrons bounce against the central QR antidot, which strongly reduces injection efficiency. Thanks to an analogy with Rutherford scattering, we show that a local partial depletion of the QR close to the edge of the antidot can counterintuitively ease ballistic electron injection. In contrast, local charge accumulation can focus the semiclassical trajectories on the hard-wall potential and strongly enhance reflection back to the lead. Scanning gate experiments on a ballistic QR and simulations of the conductance of the same device are consistent and agree that the effect is directly proportional to the ratio between the strength of the perturbation and the Fermi energy. Our observation fits the simple Rutherford formalism in two dimensions in the classical limit.

The Landauer-Büttiker formalism establishes an equivalence between the electrical conduction through a device, e.g., a quantum dot, and the transmission. Guided by this analogy we perform transmission measurements through three-port microwave graphs with orthogonal, unitary, and symplectic symmetry, thus mimicking three-terminal voltage drop devices. One of the ports is placed as input and a second one as output, while a third port is used as a probe. Analytical predictions show good agreement with the measurements in the presence of orthogonal and unitary symmetries, provided that the absorption and the influence of the coupling port are taken into account. The symplectic symmetry is realized in specifically designed graphs mimicking spin-1/2 systems. Again a good agreement between experiment and theory is found. For the symplectic case the results are marginally sensitive to absorption and coupling strength of the port, in contrast to the orthogonal and unitary case.

Recently the Luttinger field approach was proposed by Eich et al. in Phys. Rev. B 90, 115116 (2014). as a way to simulate switching on a temperature gradient across a nanoscale device initially in thermal equilibrium. The time-dependent particle and heat currents can then be calculated by propagating the initial equilibrium state of the system in time. Unfortunately, applying a uniform Luttinger field in the whole of a lead causes a discrepancy between steady-state currents in the long time limit and those predicted by the Landauer-Büttiker formulas as well as artifacts at short times. Here we propose a modified approach where the Luttinger field gradually reaches its final value across a transition region. If the length of the transition region is sufficient, the electrons move through without reflecting. In this way the correct energy distribution of electrons originating from such a lead, corresponding to the new temperature, is established in the scattering region. Our approach is tested on a single quantum dot and a parallel double quantum dot system.

We present a prediction of the Dirac semimetal (DSM) phase in ${\mathrm{MgTa}}_2{\mathrm{N}}_3$ based on first-principles calculations and a symmetry analysis. In this material, the Fermi level is located exactly at the Dirac point without additional Fermi surface pockets. The band inversion associated with the Dirac cone involves the $d$ orbitals of two structurally inequivalent Ta atoms with octahedral and trigonal prismatic coordination spheres. We further show that the lattice symmetry breaking can realize topological phase transitions from the DSM phase to a triple nodal point semimetal, Weyl semimetal, or topological insulator. The topologically protected surface states and the nonprotected Fermi arc surface states are also studied.

We report the nanoscale electrical imaging results in hexagonal ${\mathrm{Lu}}_{0.6}{\mathrm{Sc}}_{0.4}{\mathrm{FeO}}_3$ single crystals using conductive atomic force microscopy and scanning microwave impedance microscopy. While the dc and ac response of the ferroelectric domains can be explained by the surface band bending, the drastic enhancement of domain wall (DW) ac conductivity is clearly dominated by the dielectric loss due to DW vibration rather than mobile carrier conduction. Our work provides a unified physical picture to describe the local conductivity of ferroelectric domains and domain walls, which will be important for future incorporation of electrical conduction, structural dynamics, and multiferroicity into high-frequency nanodevices.

It is well known that theorems of Lieb-Schultz-Mattis type prohibit the existence of a trivial symmetric gapped ground state in certain systems possessing a combination of internal and lattice symmetries. In the continuum description of such systems, the Lieb-Schultz-Mattis theorem is manifested in the form of a quantum anomaly afflicting the symmetry. We demonstrate this phenomenon in the context of the deconfined critical point between a Neel state and a valence bond solid in an $S=1/2$ square lattice antiferromagnet and compare it to the case of $S=1/2$ honeycomb lattice where no anomaly is present. We also point out that new anomalies, unrelated to the microscopic Lieb-Schultz-Mattis theorem, can emerge, prohibiting the existence of a trivial gapped state in the immediate vicinity of critical points or phases. For instance, no translationally invariant weak perturbation of the $S=1/2$ gapless spin chain can open up a trivial gap even if the spin-rotation symmetry is explicitly broken. The same result holds for the $S=1/2$ deconfined critical point on a square lattice.

In this study, we demonstrate that optical magnetic circular dichroism (OMCD) in Faraday geometry can serve as an effective means of characterizing the valence electronic structure of magnetic oxide systems. Molecular-beam-epitaxy-grown magnetite thin film single crystals served as the test sample. The dominant spin channels of optical charge transfer were resolved from transmitting OMCD spectra, which can only be interpreted by considering electron correlation effects and including polarized $2p$ oxygen. First-principles calculations based on density-functional theory with Hubbard-$U$ correction ($\mathrm{DFT}+U$) were performed on cubic inverse spinel ${\mathrm{Fe}}_3{\mathrm{O}}_4$ ($\mathrm{F}d\overline{3}m$). We determined that the main features of optical conductivity [S. K. Park et al., Phys. Rev. B 58, 3717 (1998)] were similar to those of a Mott-Hubbard insulator. According to the extent of $2p$ character in the mixed $\mathrm{Fe}\left(3d\right)-\mathrm{O}(2p$) valence that engendered the relaxation of Laporte selection, we classified dominant optical charge transfer into three categories: (1) intervalence spin-minority $d-d$ charge transfer relax from lattice distortion; (2) intersublattice $d-d$ charge transfer across the valence gap of spin majority; and (3) ligand-to-metal $p-d$ charge transfer across the valence gap of spin minority. We conclude that the transmitting OMCD spectrum can generally reflect the competition between optical transitions from the $B$-site $\mathrm{Fe}(3d$) spin majority and the $\mathrm{O}(2p$) spin minority. Finally, we found the OMCD signal of magnetite exhibited similar trend to the valence band spin polarization deduced from Mott spin polarimetry. Excitation spectrum that access the direct information about the “bare” electronic states from soft x-ray spin-resolved photoemission were also revisited.

We study the formation of exceptional points (EPs) in a photonic medium with a complex time-periodic permittivity, i.e., $\varepsilon \left(t\right)={\varepsilon }_o+{\varepsilon }_{r}sin\left(\mathrm{\Omega }t\right)$. We formulate Maxwell's equations in the form of a first-order non-Hermitian Floquet Hamiltonian matrix and solve it analytically for the Floquet band structures. In the case when ${\varepsilon }_r$ is real, to the first order in ${\varepsilon }_r$, the band structures show a phase transition from an exact phase with real quasienergies to a broken phase with complex quasienergies inside a region of wave-vector space, the so-called $k$ gap. We show that the two EPs at the upper and lower edges of the $k$ gap have opposite chiralities in the stroboscopic sense. By picking up the mode with a positive imaginary quasienergy, the wave propagation inside the $k$ gap can grow exponentially. In three dimensions, such pairs of EPs span two concentric spherical surfaces in the $\vec{k}$ space and repeat themselves periodically in the quasienergy space with $\mathrm{\Omega }$ as the period. However, in the case when ${\varepsilon }_r$ is purely imaginary, the $k$ gap disappears and gaps in the quasienergy space are opened. Our analytical results agree well with finite-difference time domain simulations. To second order in ${\varepsilon }_r$, additional EP pairs are found for both the cases of real and imaginary ${\varepsilon }_r$. We also extend our theory to the case where the permittivity is simultaneously modulated in both space and time, i.e., $\varepsilon (x,t)={\varepsilon }_o+{\varepsilon }_{r}sin(\mathrm{\Omega }t-\beta x+\varphi )$. For a real ${\varepsilon }_r$, we find that EPs also exist when the modulation speed $c_m=\mathrm{\Omega }/\beta$ is faster than the speed of the wave travelling inside the medium.

We analyze time-dependent transport through a quantum dot with electron-electron interaction that is statically tunable to both repulsive and attractive regimes, or even dynamically driven. Motivated by the recent experimental realization [A. Hamo et al., Nature (London) 535, 395 (2016)] of such a system in a static double quantum dot we compute the geometric pumping of charge in the limit of weak tunneling, high temperature, and slow driving. We analyze the responses for all possible pumping experiments or “driving protocols”, each defined by choosing a pair of driving parameters (gate voltage, bias voltage, tunnel coupling, electron-electron interaction). We show that such responses for different experiments can be governed by a common, underlying pumping mechanism, which is characterized by a set of effective parameters. The latter are nontrivial combinations of the experimentally driven parameters and other static parameters. If two different pumping experiments result in the same modulation of the effective parameters, i.e., the underlying mechanism is the same, then their responses will also be the same. Interestingly, for static attractive interaction we find a nonzero pumping response despite the attractive Coulomb blockade that hinders stationary transport. Furthermore, we identify a unique pumping response whose underlying mechanism relies on the interaction to be one of the driving parameters: it cannot be obtained with other sets of driving parameters. Finally, although a single-dot model with orbital pseudospin describes most of the physics of the mentioned experimental setup, it is crucial to account for the additional (real-)spin degeneracy of the double dot and the associated electron-hole symmetry breaking. This is necessary because the pumping response is more sensitive than dc transport measurements and detects this difference through pronounced qualitative effects.

Angle-resolved, mono- and bichromatic two-photon photoemission was used to study the electronic structure at the surface of the giant Rashba system Bi/Ag(111). The photoemission intensity maps are exceptionally rich in structure due to a large manifold of initial and intermediate states that allow for various resonant interband transitions. Using different photon energies, we succeeded in disentangling the complex experimental data. Close to the Fermi level the unoccupied, Rashba-split $p_x{p}_y (m_j=1/2)$ surface state can be identified with a Rashba parameter of $3.0\pm 0.2$ eV Å and a lifetime of $26\pm 5$ fs. At higher energies the spin-split $p_x{p}_y (m_j=3/2)$ and $p_z$ bands as well as the first three members of a series of image-potential resonances were observed. Circular dichroism was used to gain information on the spin structure of both the initial and intermediate states and to corroborate the assignment to Rashba bands.

Using first-principles calculations, we study the dependence of the electronic and vibrational properties of multilayered ${\mathrm{PbI}}_2$ crystals on the number of layers and focus on the electronic-band structure and the Raman spectrum. Electronic-band structure calculations reveal that the direct or indirect semiconducting behavior of ${\mathrm{PbI}}_2$ is strongly influenced by the number of layers. We find that at 3L thickness there is a direct-to-indirect band gap transition (from bulk-to-monolayer). It is shown that in the Raman spectrum two prominent peaks, $A_{1g}$ and $E_g$, exhibit phonon hardening with an increasing number of layers due to the interlayer van der Waals interaction. Moreover, the Raman activity of the $A_{1g}$ mode significantly increases with an increasing number of layers due to the enhanced out-of-plane dielectric constant in the few-layer case. We further characterize rigid-layer vibrations of low-frequency interlayer shear (C) and breathing (LB) modes in few-layer ${\mathrm{PbI}}_2$. A reduced monoatomic (linear) chain model (LCM) provides a fairly accurate picture of the number of layers dependence of the low-frequency modes and it is shown also to be a powerful tool to study the interlayer coupling strength in layered ${\mathrm{PbI}}_2$.

By imposing the constraints of structural compatibility, stability, and a large tunneling magneto-resistance, we have identified the ${\mathrm{Fe}}_3\mathrm{Al}/{\mathrm{BiF}}_3/{\mathrm{Fe}}_3\mathrm{Al}$ stack as a possible alternative to the well-established FeCoB/MgO/FeCoB in the search for a novel materials platform for high-performance magnetic tunnel junctions. Various geometries of the ${\mathrm{Fe}}_3\mathrm{Al}/{\mathrm{BiF}}_3/{\mathrm{Fe}}_3\mathrm{Al}$ structure have been analyzed, demonstrating that a barrier of less than 2 nm yields a tunneling magneto-resistance in excess of 25 000% at low bias, without the need for the electrodes to be half-metallic. Importantly, the presence of a significant spin gap in ${\mathrm{Fe}}_3\mathrm{Al}$ for states with ${\mathrm{\Delta }}_1$ symmetry along the stack direction makes the TMR very resilient to high voltages.

We investigate the field-induced crossover between two types of isolated skyrmions that exist within the conical phase of cubic helimagnets and orient themselves either along or perpendicular to the field. Such a crossover takes place for the same value of the field at which the closely packed skyrmion lattice was predicted to stabilize in the $A$-phase region. The clusters and a skyrmion lattice composed by the skyrmions perpendicular to the field, however, are unfavorable and lose their stability as compared with the skyrmions parallel to the field. We also followed the transformation of perpendicular skyrmions into pairs of merons that rupture the helical state. Attractive interactions between different types of isolated skyrmions make it feasible to construct complex cluster states with the cubic arrangement of skyrmions.